Co2 desorption without stripper

ABSTRACT

A CO 2  desorption method and equipment for performing this method is described. More specific, a method for desorbing CO 2  from an absorption fluid without involving a traditional stripper but instead using a heat exchanger as a flash-tank is disclosed. Further described is the utilization of cooling heat from a condenser for cooling the lean absorbent fluid.

This is a divisional application of U.S. patent application Ser. No.13/701,764, which is a 35 USC §371 national stage application ofPCT/EP2011/059076, which was filed Jun. 1, 2011, both of which areincorporated herein by reference as if fully set forth.

The present invention relates to a CO₂ desorption method and system forperforming this method. More specific the present invention relates to amethod for desorbing CO₂ from an absorption fluid without involving atraditional stripper.

The separation of CO₂ from exhaust gases in connection withpost-combustion power plants has received much attention lately and thepresent invention is especially applicable in connection with desorptionof CO₂ captured from an exhaust gas stream.

In existing technology for CO₂ capture, a counter-current packed columnstripper and a reboiler is often an integral part of the desorptionprocess (stripping). In this process, chemical bound CO₂ in the liquidphase (typically an amine solution) is released into the gas phase. Thisis done by adding heat (typically steam) to obtain proper temperatureand pressure for desorption in the stripper and the reboiler. Animportant conception is the CO₂ loading, defined as moles of CO₂ inliquid per moles of amine in liquid. Lean amine enters the absorber andleaves the stripper. Rich amine leaves the absorber and enters thestripper. As the names indicate the rich amine solution contains moreCO₂ than lean amine, and therefore, has higher loading. The loadingdepends strongly on the liquid temperature. Some of the amine, CO₂ andvapour released in the reboiler are transported back to the bottomsection of the stripper column, making the reboiler an integral part ofthe stripper column A condenser is installed to receive the stream thatleaves the top of the stripper and remove water and absorbent.

A disadvantage with the existing design is that cold water with aminefrom the condenser is returned to the top of the stripper column andmixed with the rich amine, making the rich amine even colder.

Existing technology is expensive, and simplification is required inorder to reduce energy consumption and building costs.

In U.S. Pat. No. 4,553,984, a process for the separation of CO₂ from ahydrocarbon gas stream is disclosed, the desorption proceeds in a flashtank. Heat is supplied to the rich absorbent stream in a separate heatexchanger arranged up stream of the flash tank. Water that leaves overthe top of the flash tank is replaced by steam supplied to the bottom ofthe flash tank.

U.S. Pat. No. 5,820,837 discloses a process for removing the acid gassesCO_(s) and H₂S from a hydrocarbon gas stream. The disclosed processinvolves two flash steps. The first removes absorbed hydrocarbons;thereafter, the rich absorbent is heated in a heat exchanger before CO₂is flashed of. The heating accordingly occurs in a separate unitarranged upstream the flash unit. Further a stripper is included toremove remaining gas. Condensed solvent is returned to the top of thestripper.

U.S. Pat. No. 5,061,465 discloses a process for removing CO₂ from ahydrocarbon gas stream. The process comprises absorption and desorptionutilizing a series of flash tanks Supplying heat directly in the flashtanks is not disclosed.

The aim of the present invention is to provide a simpler solution forstripping a rich absorbent fluid that requires less equipment. A furtherobject is to provide a method which is more energy efficient.

The present invention provides a method for desorbing carbon dioxidefrom a CO₂ rich absorption fluid comprising a CO₂ absorbent, a solventand absorbed CO₂, characterised in that the method comprises

-   -   heating the CO₂ rich absorption fluid,    -   expanding the heated CO₂ rich absorption fluid in a flash tank,    -   separating the expanded absorption fluid in a vapour phase        comprising desorbed CO₂ and vaporised solvent and a main liquid        phase comprising CO₂ lean absorption fluid,    -   condensing at least part of the solvent within the vapour phase,        and [0015] mixing the condensed solvent with the main liquid        phase.

By mixing the condensed vapour phase mainly comprising solvent with themain liquid phase the cooled lean absorption fluid stream is obtained.Accordingly, the present invention provides the utilization of coolingheat from a condenser for cooling the lean absorbent fluid.

In one embodiment of the method the difference in CO₂ loading betweenthe rich and lean amine stream is within the range 0.20-0.30 molCO_(2/)mol absorbent, preferably between 0.23-0.27 mol CO_(2/)molabsorbent. With a difference between rich and lean amine loading of theorder 0.2-0.3 in both absorber and stripper, the CO₂ removal efficiencywill normally be 85 mol % or above, also depending on correct gas/liquidflow ratio in the absorber.

The method according to the present invention may further compriserecycling a part of the main liquid phase into the rich absorption fluidprior to the mixing with condensed solvent and optionally heating a partof the main liquid phase before returning it to the flash tank.

Another aspect of the present invention is a system for desorbing carbondioxide from a CO₂ rich absorption fluid, characterised in that thesystem comprises a flash-tank with an inlet, a vapour outlet and liquidoutlet, where the vapour outlet is in fluid communication with an inletto a condenser, where the condenser has a condensed fluid outlet influid communication with the liquid outlet from the flash-tank.

In one embodiment of the system it further includes a system forsupplying heat to the flash-tank.

In yet another embodiment the flash-tank is a reboiler, a stratifiedflow pipe or a fired heater combined with a vessel.

The system according to the present invention may in one embodimentfurther comprise a recycling pipeline in fluid communication with theliquid outlet and an inlet to the flash tank, arranged up-stream thefluid communication between the condensed fluid outlet and the liquidoutlet.

With the present invention both building cost and operating cost will bereduced compared to existing technology, mainly due to increased processsimplicity. Simplicity is also favourable with respect to saferoperation.

The idea of mixing the cold liquid flow from the condenser with the leanabsorbent flow from the “flash-tank” is favourable with respect toreduced energy consumption. The rich absorbent shall not be cooled. Onthe contrary, the temperature of the rich absorbent in the presentinvention shall preferably be raised compared to today's practice.

The present invention will be described in further detail with referenceto the enclosed figures where:

FIG. 1 illustrates a desorption process according to the presentinvention.

FIG. 2 illustrates another embodiment of the process illustrated in FIG.1.

FIG. 3 illustrates a second embodiment of the present invention wherethe flash unit is a heated, stratified flow pipe.

FIG. 4 illustrates a third embodiment of the present invention where theflash unit is a fired heater and a vessel.

However it should be understood that the figures are enclosed forillustration purposes and that the scope of the present invention is notlimited to the illustrated embodiments.

The present invention is in the present description illustrated byreferring to MEA (MonoEtanholAmine) as absorbent, however it should beunderstood that the present invention is equally applicable for use withother amines and especially for absorbents where the binding energy ofCO₂ in the absorbent is less than for MEA.

The temperature and pressure for the flash must in each case be selectedaccording to the chosen absorbent.

In the prior art solution, the purpose of the stripper and the reboileris to release CO₂ from a rich amine liquid into a vapour phase in directcontact with the liquid. The stripper column in existing technologyoperates with a lower temperature of the rich amine entering at the topof the column compared to the temperature of the lean amine outlet atthe bottom of the stripper. However, if the temperature of the richamine is increased from typically 110° C. to about 120° C. at about 2bar absolute more CO₂ will immediately be released at the stripper inletmaking the loading at the top of the column approximately equal to thelean amine loading at the bottom. Hence, the stripper column may besuperfluous, or at least, the packed column height can be considerablyreduced, since much CO₂ is flashed off before the liquid starts flowingdown the column

The main idea of the invention is to keep the inlet temperature andpressure of the rich amine at such values that the result after theflash is a moderate vapour fraction and a liquid with a specified leanamine loading. This is done by tuning the process with respect to liquidtemperature and specified lean amine loading. The flash requires a“flash-tank”, which can be any appropriate heat exchanger.

The lean MEA loading after flash is shown in Table 1 for various liquidtemperatures and a typical pressure of 1.8 bara absolute. The weightpercent MEA is 30% in this example, which is common for traditionalcolumn strippers.

Table 1 illustrates how a higher temperature results in desorption ofmore CO₂ from MEA during a flash process.

TABLE 1 Lean amine loading as function of inlet liquid temperature andpressure for 30 w % MEA, using Keng-Eisenberg equilibrium method FlashRich calc. Fluid Pressure amine loading (mol Lean amine loading no.temp. ° C. (bara) C02/mol MEA) (mol C02/mol MEA) 1 109.0 1.8 0.42 0.38 2115.0 1.8 0.42 0.31 3 120.0 1.8 0.42 0.17

There are several candidates for the “flash-tank” as described later on.The CO₂, vapour, and amine released in the flash immediately leave the“flash tank” at the top. The gas mixture is then cooled and separated ina condenser as in existing technology. The separated colder water andamine mixture may then be mixed with the lean amine from the “flashtank” to maintain the liquid circulation rate and w % of amine.

When flashing the specified rich amine in a “flash tank” at a highertemperature T and at an appropriate pressure P, the result after theflash is a moderate vapour fraction and a predefined lean amine loading.

The present method is flexible with respect to getting specified valueof lean and rich amine loading. This is controlled by using correcttemperature and pressure.

The “flash tank” in this context is any heat exchanger that makes thestripper column superfluous or at least reduced in size. Below someexamples of heat exchangers are given. The first idea is to let the“flash tank” be an existing or a modified kettle reboiler. Thisembodiment is illustrated in FIG. 1. Here rich amine 10 passes through apump 12 and valve 14 and enters the reboiler 18 via stream 16. Flashingof the rich amine results in a vapour phase comprising CO₂ and solventbut which may also comprise a part of the absorbent. The vapour phase ispassed through stream 24 to a condenser 26, where the solvent and theabsorbent are condensed whereas the CO₂ stays in the gas phase and iswithdrawn through stream 28. The condensate is passed through stream 30and combined with the stream of lean absorbent 36 from the reboiler 18.The stream 30 having been cooled in the condenser 26 will hold a lowertemperature and the combination of the two streams result in a leanabsorbent stream 34 which has a lower temperature than would have beenthe case if the condensate had been returned to the reboiler/flash-tank.A pump 38 pumps the lean absorbent into stream 40 which is connected tothe absorber (not shown). Heat is supplied to the reboiler 18 byentering steam trough stream 20 and removing steam with a reducedtemperature through stream 22. In this embodiment the traditionalstripper column has been fully eliminated which makes the system moresimple. Further the lean absorbent stream 40 may have a lowertemperature and therefore requires less cooling before it can enter theabsorber, dependent on the temperature within the reboiler.

In this example, the rich amine from the absorber enters the reboilerdirectly without first passing through a counter-current packed columnstripper. The temperature T and pressure P in the reboiler is tuned togive correct lean amine out of the reboiler. The vapour leaving thereboiler is condensed in the condenser, and the cold liquid from thecondenser is mixed with the warm lean amine from the reboiler. Thisensures that the liquid circulation rate and w % amine in the system isfairly constant. The loading of the lean amine returning to the absorbermay be slightly changed after the mix of the two liquid streams. Thecorrect temperature of the liquid in the reboiler is in this examplecontrolled by the steam consumption and steam temperature.

The condensed water with amine is, in the present invention, used tocool the lean amine which is produced in the reboiler. This is anadvantage compared to existing technology since the lean amine mustanyway be cooled before it re-enters the absorber. The lean loading isnormally not altered much after mixing of these two liquid streams.Which vapour fraction is best depends on the chosen level of the leanand rich amine and the capacity of the condenser. The completedesorption process can be optimized accordingly.

FIG. 2 illustrates a further possible aspect of the embodiment of thepresent invention illustrated in FIG. 1, but with two optional refluxesin the reboiler. Here the lean absorbent stream from the reboiler may bepartly returned either as stream 46 to the rich absorbent stream orpartly as stream 44 heated in heat exchanger 42 and returned to thereboiler as stream 48. This solution with streams 44 and 46 can be usedfor additional lean amine loading control if necessary.

In one aspect of the present invention it is possible to connect morereboilers in series, however, to lower the complexity and the number ofunits it is considered advantages to use only one reboiler.

In FIG. 3, a second embodiment of the present invention is shown. Herethe reboiler utilized in the embodiments illustrated in FIG. 1 and hasbeen replaced by a heated, stratified flow pipe 17. The rich absorbentstream 16 enters the pipe 17 at one end. Within the pipe the flashingoccurs, and the stream is split into a liquid phase 25 and a vapourphase 23. Heat 21 is supplied to the pipe from the outside. At the endof pipe 17 vapour phase is directed into line 24 and into a condenser 26similar to the embodiments illustrated on FIGS. 1 and 2. The leanabsorbent is directed as stream 36, mixed with the condensate in stream30 and returned to the absorber (not shown) via stream 40. Some leanamine may optionally be recycled through stream 46 to the pipe inlet,possibly with heating of the recycled stream (not shown). The conditionsfor obtaining stratified flow in pipes are well known in the art and canbe found in flow regime charts in handbooks or text books. The pipe canbe coiled if that is practical for space reasons or other reasons.Optionally the cross-sectional area of the pipe can vary with axialposition to obtain best performance. The cross-sectional area can beelliptic, circular or any other suitable shape.

The embodiment illustrated in FIG. 4 is similar to the embodiment shownin FIG. 1 except that the reboiler has been replaced by a fired heater19 and a vessel 33. The rich absorbent in stream 16 enters the firedheater where external heat is added. The heating and flashing within thefired heater 19 transports the fluid into the vessel 33. Here the fluidis separated into a vapour phase which is transferred over stream 24 tothe condenser 26. The liquid lean absorbent phase in vessel 33 isremoved through stream 36 at the bottom. It can be mixed with thecondensate in stream 30 and returned to the absorber (not shown) viastream 40. The vessel 33 may be equipped with a liquid control unit 32.The CO₂ gas is removed through stream 28. If the lean amine loading fromthe first flash tank is higher than specified, two or more fired heaterswith flash tanks can be connected in series. Some lean absorbent mayoptionally be recycled through line 46 to the fired heater 19 with orwithout heating of the recycled stream.

The figures illustrate some embodiments of the “flash tank” according tothe present invention but also other units can be applied. The “flashtank” in this context can be any appropriate heat exchanger with richabsorbent kept at appropriate T and P. For example, the flash tank canbe a fired heat exchanger (fired reboiler), a tank with heating coils(with steam or another heat source), a thermosyphon reboiler(circulation driven by differences in density), a pipe transporting richamine and heated to correct temperature and pressure, where gas and leanamine is separated by flashing the rich amine liquid along the pipe orany other appropriate heat exchanger. An example of the latter is wherethe flash appears in a spiral coil tube or in parallel spiral coil tubesrather than in a straight pipe. In that case, the spiral coil or coilscan for example be wound on a solid cylinder. A spiral coil has theadvantage of obtaining a long flash pipe with reduced height or lengthcompared to a straight pipe.

EXAMPLE

As an example of the idea, a flash calculation using HYSYS processsimulator, indicates that a temperature of about 120° C. and a pressureof about 1.8-2 bars absolute are appropriate in order to get a typicalvalue of the lean amine loading. Other (T,P) combinations are possibledepending on the wanted lean and rich amine loading and the maximumvapour fraction one allows after the flash. Some processes are based ona low value for lean amine loading (typically 0.16) and a moderate valuefor rich amine loading (0.36). Other processes are based on a high valuefor lean amine loading (typically 0.23) and a rich amine loading(typically 0.45).

What is claimed is:
 1. A system for desorbing carbon dioxide from a CO₂rich absorption fluid, the system comprising a flash-tank with an inlet,a vapour outlet and liquid outlet, where the vapour outlet is in fluidcommunication with an inlet to a condenser, where the condenser has acondensed fluid outlet in fluid communication with the liquid outletfrom the flash-tank.
 2. A system according to claim 1, wherein thesystem further includes a system for supplying heat to the flash-tank.3. A system according to claim 1, wherein the flash-tank is a reboiler,a stratified flow pipe, or a fired heater combined with a vessel.
 4. Asystem according to claim 1, wherein the system further comprises arecycling pipeline in fluid communication with the liquid outlet and aninlet to the flash tank, arranged up-stream the fluid communicationbetween the condensed fluid outlet and the liquid outlet.